Dynamically vulcanized thermoplastic elastomers (thermoplastic vulcanizates, or “TPVs”), as with traditional thermoplastic elastomers, have a combination of both thermoplastic and elastic properties. The thermoplastic vulcanizates are prepared by mixing and shearing a thermoplastic polymer, a vulcanizable elastomer and a curing agent. The vulcanizable elastomer is dynamically cured during the shearing and mixing and is intimately and uniformly dispersed as a particulate phase within a continuous phase of the thermoplastic polymer. See, for example U.S. Pat. Nos. 4,130,535, 4,311,628, 4,594,390 and 6,147,160. Typically, the thermoplastic forms a “continuous” phase in the TPV, which is desirable for later processing the TPV into articles of manufacture. What would be desirable is to find a balance of the elastomer and thermoplastic in TPVs such that a higher degree of elastomer relative to thermoplastic can be achieved in order to make soft articles.
The physical properties of TPVs, in particular, the “softness” or “hardness”, measured quantitatively as Shore A (or D) Hardness, are controlled by many factors. One factor is the balance of elastomer versus thermoplastic that is present in the TPV. Increasing the relative amount of elastomer tends to make the TPV softer, while increasing the relative amount of thermoplastic makes the TPV harder. These physical characteristics are, however, in balance with the processability of the TPV, that is, how well it extrudes to form articles of manufacture. Lowering the level of thermoplastic below a certain threshold can often result in a loss of the continuous thermoplastic phase and agglomeration of the elastomeric phase, making any finished article having a “powdery” quality. Also, this lowered level of thermoplastic will diminish the processability of the TPV. This diminished processability can be compensated for by addition of oils to the elastomer/thermoplastic blends. However, the addition of excessive oils can lead to a disproportionate swelling of the elastomer phase of the TPV, thus reducing the thermoplastic phase volume.
The cure state of the TPV is another factor that may independently influence the softness or hardness of the TPV. Some in the art have found that lowering the cure state of the TPV can result in a TPV that is relatively “soft” without lowering the amount of “hard” phase in the TPV. Yet, at least in some situations, a soft and fully cured TPV is desired. Since the elastomer and the oil are the soft components, increasing these components can be viewed as a way of making a “soft” TPV. Achieving this, while maintaining the fully-cured nature of the elastomeric portion of the TPV, would be desirable.
Soft thermoplastic vulcanizates are described in EP 0 892 831, those thermoplastic elastomers have a hardness of less than 35 Shore A. The patentee demonstrates some TPVs having these “soft” Shore A values that possess at least 30 wt % thermoplastic. The softness is achieved at least in part due to the presence of amorphous thermoplastic in the TPV. Without this, the low level of thermoplastic alone results in products that are unprocessable. This is recognized in the art, such as in EP 0 109 375 B2, where in discussing the relative proportions of cured elastomer and plastic in dynamically vulcanized thermoplastic elastomers, the maximum amount of elastomer (i.e., the lowest amount of thermoplastic) is where the product crumbles and no longer retains the form of a mass.
TPVs comprising a thermoplastic polyolefin and a dynamically vulcanized elastomer having a low thermoplastic content are illustrated to have a hardness less than 35 Shore A in WO 2005/010094 Al where the oil to elastomer ratio is at least 2/1. The examples illustrate a lowest level of the polypropylene thermoplastic at 5.12 wt % based upon the total weight of the thermoplastic elastomer composition, or 18.7 wt % on a total polymer basis (total of thermoplastic and elastomer). It also shows that with the lowest amounts of curative agent (least cure) the softest composition are obtained, and that higher levels of curative agent (greater cure) produce compositions with hardness of above 25 Shore A. The examples at higher levels of cure contained a total of 300 phr oil. This is consistent with the discussion by T. Abraham, C. McMahan in Thermoplastic Elastomers: Fundamentals and Applications, RUBBER COMPOUNDING: CHEMISTRY AND APPLICATIONS 163, 206-209 (Marcel Dekker, 2004), which illustrates the conversion of thermoplastic olefin blends (TPO) containing polypropylene, ethylene-propylene-diene monomer rubber (EPDM) and oil, into thermoplastic vulcanizates (TPV) by cross-linking the elastomer. Though many elastomeric properties are improved, the hardness (Shore A) of the compositions is shown to increase with each conversion. This hardness change (increase) is thought to be dependent on the change in morphology of the product and on the cross-linking of the elastomer phase. See also, A. Y. Coran, in Vulcanization, SCIENCE AND TECHNOLOGY OF RUBBER, 291-92 (Academic Press, F. R. Eirich, ed., 1978) (especially FIG. 1) where hardness for vulcanized elastomer is illustrated to increase with the crosslink density, that is, state of cure.
In view of the expanding use of thermoplastic elastomers, both thermoplastic vulcanizates and others (e.g., styrenic triblock thermoplastic elastomers), many efforts are being expanded to find those with best properties and produced using the most efficient manufacturing means. Considering the desirability of TPVs that are substantial cured, it would be advantageous to have a process for making TPV compositions also having a hardness on the Shore A equal to or lower than 25, yet being free of “powder”, that is, pellets that are non-agglomerating and thus suitable for subsequent plastics processing.